Generally, a wind turbine includes a wind tower, a nacelle mounted on the wind tower, and a rotor coupled to the nacelle. The rotor generally includes a rotatable hub and a plurality of rotor blades coupled to and extending outwardly from the hub. Each rotor blade may be spaced about the hub so as to facilitate rotating the rotor to enable kinetic energy to be converted into usable mechanical energy, which may then be transmitted to an electric generator disposed within the nacelle for the production of electrical energy. Typically, a gearbox is used to drive the electric generator in response to rotation of the rotor. For instance, the gearbox may be configured to convert a low speed, high torque input provided by the rotor to a high speed, low torque output that may drive the electric generator.
Most wind turbines include support towers that include a plurality of tubular-shaped tower support sections. Adjacent tower support sections are coupled at structural connections using welding and/or mechanical fastening of ring flanges to form support tower assemblies. Wind towers are subject to large cyclic loading, which results in a large displacement of tower support sections and increased bending stresses and torsional stresses induced to the tower support members. The flanges of the structural connection between tower sections are preloaded by the bolted connections, such that a compressive stress is generated under the bolt head and nut, which exceeds any fluctuating loads experienced by the tower connection under functional loads, including generator reactive torque, gyroscopic loads due to change of direction of the turbine rotational axis, and dynamic loads due to imbalance or resonance. The mating flange faces are loaded under the nut and bolt, with the loading being relaxed between bolted connections. Axial loads transmitted through the tower about an axis parallel with the vertical axis of the tower are resisted by the friction generated between the flange faces under the clamping load of the bolts by the coefficient of friction between the flanges.
Some support tower members may be subjected to stresses that cause fatigue cracking and/or failure, particularly at the joint between adjacent support tower members and between the tower top and the yaw bearing. The primary mode of failure in the structural connections of wind tower joints can be bolt failure by the compromise of bolt preload. The bolts begin to experience fluctuating loads and stresses once the bolt preload is reduced, and this fluctuating load leads to fatigue failure of the bolt as well as fatigue cracking of nearby steel. Conventional methods for repairing support tower members include disassembling the entire support tower, replacing support tower members, and reassembling the support tower, which is expensive and time consuming.